1. Field of the Invention
The present invention relates to acoustic apparatus and methods for fluid flow measurement. More specifically, it relates to ultrasonic time of flight sensors configured as insertion probes.
2. Background Information
It is known in the flow measurement arts to configure a time-of-flight sensor comprising a pair of probes spaced apart along a flow direction, as shown schematically in FIG. 1. Generally, each of these probes has an ultrasonic transducer set perpendicular to the flow direction and facing another transducer on the other probe so as to define an acoustic beam extending along the flow direction. This arrangement samples only that portion of the fluid lying directly between the two probes.
Conventionally, these transducers constitute a flat face on the downstream side of the upstream probe and another flat face on the upstream side of the downstream probe. The bluff downstream face on the upstream probe is particularly problematic in that it facilitates the generation of vortices that can intercept the acoustic beam.
In his U.S. Pat. Nos. 7,044,000 and 7,201,065, the inventor provides ultrasonic time-of-flight flow sensors using quasi-helical acoustic beams that sample a greater fraction of the flowing fluid. These beams may extend between two transducers spaced apart along a flow direction and skewed with respect to that flow direction so as to reflect the beam off the inside walls of the pipe. In an embodiment depicted in FIG. 2, the two transducers are mounted on a single probe head. A drawback of this arrangement is that mounting the two transducers on a single probe head increases the effective diameter of the probe, thus requiring a larger fitting and a larger pipe opening for inserting the probe into the pipe. Another drawback is that the acoustic path length is less than what could be provided if the transducer spacing along the flow direction was increased beyond the limit set by the largest feasible fitting.